1. Field of the Invention
This invention relates generally to hollow structural members and more specifically to hollow utility poles having tapered external dimensions and substantially uniform wall thickness.
2. Description of the Prior Art
The geometric shape of tapered poles in high stress, high lateral-force environments typically encountered in the utility industry inherently provides the greatest cross-sectional strength at or near the base of the pole where reactive forces tend to combine and converge.
Problems associated with the use of wooden poles within the utility industry are numerous and well documented. These problems include ground line decay, the tendency of wooden poles to bend or arc under high lateral forces (thereby requiring the use of guying cables, anchors and other auxiliary hardware), deterioration caused by birds and other animals, vibration damage to attached hardware and fixtures, shortages of suitable trees for producing large poles, and high breakage rate due to natural and manmade forces. Additionally, wooden poles as a general rule must be replaced after approximately 20-30 years of service. Of particular note, replacement and disposal of wooden poles is a mounting, exorbitant liability for companies, especially when these poles cannot be incinerated due to regulatory mandates. Further, some utility companies have resorted to wrapping their wooden poles in wire mesh to prevent damage to the poles by birds and other animals.
Other companies have resorted to using hollow concrete poles, particularly with regard to high voltage electric transmission systems, in order to overcome some of the difficulties associated with wooden poles. Concrete utility poles, however, are expensive to produce, are very heavy and require special heavy-duty equipment to load, transport, unload and install. Further, due to strength requirements, concrete poles are usually solid at the top, where the pole has its smallest diameter. Moreover, field modifications of concrete poles are generally very time consuming, cumbersome and thus very costly.
Steel utility poles are currently available in the market. Steel poles are typically produced by cutting thin and malleable steel plates and forming the plates, using metal brakes and other hot and/or cold metal shaping methods, into two multi-sided halves which are then welded together. Such poles are quite expensive to produce and, as a consequence of the thin walls necessary to form them, result in poles having relatively large base diameters in order to achieve the necessary strength to be used in high stress, high lateral-force situations such as those subject to utility poles. Further, wall thickness can be varied only sectionally, if at all.
Aluminum alloy utility poles are also available. However, these poles are generally appropriate for low stress, single-use application, such as highway lighting. Further, aluminum poles are not currently formed by centrifugal casting methods and are typically produced by hot or cold metal-shaping methods. Moreover, aluminum poles do not have the necessary qualities or strength to be used in high lateral-force environments, such as those to which utility poles are subjected.
Ductile iron has characteristics which are ideal for use in the utility pole environment. For example, ductile iron has a superior strength/weight ratio, is machineable, resilient (as opposed to brittle), has favorable damping properties, is rust proof and is cheaper than aluminum and concrete. Therefore, if centrifugally cast into geometric shapes necessary for utility pole applications, ductile iron would provide a virtually maintenance free, extremely long life (many 100+ year old ductile iron water mains are still in use), low cost utility pole which effectively overcomes the problems and difficulties associated with other utility poles as discussed hereinabove.
Centrifugal casting offers additional benefits over alternative production techniques such as metal forming. For instance, centrifugal casting produces a seamless pole, which ensures overall strength in high stress, high lateral-force environments. Furthermore, centrifugal casting allows a large volume of poles to be produced inexpensively through a mechanized process involving little human labor. Since long, seamless sections can be cast in a single pass, subsequent joining procedures during manufacture or field installation are reduced or eliminated. Also, centrifugal casting produces a much more precise pole than that created through metal forming, and is capable of producing poles within tolerances of 0.001 mm. In summary, centrifugal casting produces stronger, less labor-intensive poles that are more easily mass produced for a wider variety of applications.
Prior to the present invention, centrifugal casting of tapered metal utility poles (including ductile iron) has been limited by the casting technology. Single-speed casting methods (such as that disclosed in Applicant's prior U.S. Pat. No. 5,784,851) prevented the metal infusion from dispersing properly such that the amount painted on the inside of the mold as it spins and travels can be deposited end-to-end uniformly to high tolerances (0.001 mm.). This has meant that centrifugal casting of tapered poles was only slightly more versatile than metal rolling, though there are other advantages mentioned hereinabove. The present invention overcomes this limitation by introducing a variable-speed, variable-infusion casting method, whereby the metal-painting is precisely controlled by increasing or decreasing the metal pouring rate and the spin speed of the mold. Depending upon the desired specifications, poles can be formed with walls that are thick at bottom and thin at top, that vary in thickness at specified points, or that are substantially uniform in wall thickness along the long axis of the pole.
Further, the present invention allows production of poles that fulfill specific application requirements. For example, while some applications require variable thicknesses (thin at the top, thick at the ground line to resist line loads and corrosion), other applications may require uniform thickness for maximized structural support. The present invention also fulfills applications that require extreme or variable heights, in that the poles may be cast with slip-jointed pole sections, whereby the tapered section of one pole can be secured into the butt end of another pole to reach specified effective pole lengths. Additionally, special applications may require further enhancement of the pole during or post-casting including, but not limited to, wind-resistant texturing, mount flanges, hardware and fixture access holes or panels, and top-end caps.